add constraints for minimum length for option fields fixes 619

[ejratl]

As described in issue #619 the tables for the Windows™ PE Optional Header Type observable define minimum length requirements for both STIX 2.0 and STIX 2.1.

This pull request fixes #619 and adds the min=0 constraint for each of the fields documented as "The value of this property MUST NOT be negative."

[Copilot]

Pull request overview

This PR adds min=0 constraints to IntegerProperty fields in the WindowsPEOptionalHeaderType observable class, ensuring compliance with STIX 2.0 and 2.1 specifications that require certain properties to be non-negative.

Changes:

• Added min=0 constraint to size_of_heap_reserve and size_of_heap_commit in v21
• Added min=0 constraint to multiple size-related fields in v20 (size_of_code, size_of_initialized_data, size_of_uninitialized_data, size_of_image, size_of_headers, size_of_stack_reserve, size_of_stack_commit, size_of_heap_reserve, size_of_heap_commit)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
stix2/v21/observables.py	Adds min=0 constraint to size_of_heap_reserve and size_of_heap_commit, completing the set of non-negative constraints for WindowsPEOptionalHeaderType
stix2/v20/observables.py	Adds min=0 constraint to nine size-related fields, bringing v20 into alignment with v21 and STIX 2.0 specification requirements

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